Practice question

Role of Sonic hedgehog in splitting single eye field into bilateral domains was demonstrated by knockout resulting in cyclopia. Shh-null mice, zebrafish cyclops mutants deficient in Nodal upstream of Shh, and pharmacologic cyclopamine inhibition of Smoothened all exhibit failure to suppress midline Pax6, retaining fused eye field leading to single median eye cup, absent hypothalamus, and holoprosencephaly. Overexpression of Pax6 in midline does not split field, optic vesicle graft to trunk tests lens competence not midline, L-Maf overexpression drives crystallin. Cyclopia after SHH loss directly illustrates midline inhibitory function and organizer role.